As used herein, the term "process cheese-type products" includes those products known and referred to as "pasteurized process cheese", "pasteurized process cheese food", "pasteurized process cheese spread" and "pasteurized process cheese product". "Process cheese type-products" also includes products resembling process cheese, process cheese food, process cheese spread and process cheese product, but which may not meet the U.S. Federal Standards of Identity for any of the above products in that they contain ingredients not specified by such Standards, such as vegetable oil or vegetable protein, or do not meet the compositional requirements of such Standards. Process cheese-type products also includes any product having the flavor and texture of a process cheese-type product regardless of the ingredients or manufacturing steps employed, and regardless of whether the Standards which exist are met.
Natural cheese is generally made by taking animal milk, developing acidity and setting the milk with a clotting agent, such as rennet, or by developing acidity to the isoelectric point of the protein. The set milk is cut and whey is separated from the resulting curd. The curd may be pressed to provide a cheese block and curing takes place over a period of time under controlled conditions. A product having flavor and body characteristics of natural cheese has been made by replacing at least a portion of the animal fat of the milk with a vegetable fat, such as corn oil and/or by replacing at least a portion of the casein of the milk with an analog protein.
It is well known to provide a product having some of the characteristics of natural cheese by grinding a natural cheese, and heating it with an emulsifying salt. The name given to the resulting product depends upon the ingredients used and its composition and, in some instances, is determined by regulations promulgated by the U.S. Food and Drug Administration 21 C.F.R. .sctn.133.169-180. For example, the term "pasteurized process cheese" refers to a product comprising a blend of cheeses to which an emulsifying agent, usually an emulsifying salt, and possibly acids, are added; the mixture is then worked and heated into a homogeneous plastic mass.
The term "pasteurized process cheese food" refers to a product which is prepared from the same materials and the same processes used for manufacture of process cheese. However, cheese food generally has dairy ingredients added thereto, such as cream, milk, skimmed milk, whey or any of these from which part of the water has been removed (e.g., concentrated skimmed milk). The moisture level in process cheese food is generally higher than that of process cheese and may be up to about 44%. Fat is present at a level of not less than 23%.
The term "pasteurized process cheese spread" refers to a product which is similar to cheese food, in the sense that it can contain the indicated dairy ingredients. Process cheese spread, however, may have a moisture level as high as 60%. The minimum fat level for cheese spread is 20%.
Process cheese, process cheese food and process cheese spread are referred to as "standardized products", since their method of manufacture and composition are determined by Federal Standards of Identity.
In prior manufacture of process cheese-type products, two forms of batch cookers have been generally known, one being a steam jacketed kettle equipped with a mechanical agitator and the other being a horizontally extending cooker also known as a "lay-down" cooker having a screw in the cooker chamber. Live steam is injected directly into the chamber and the raw material at atmospheric pressure is agitated by the screw. The raw material is heated in the cooker to a temperature of at least 150.degree. F., usually about 160.degree. F. to 190.degree. F., and is held at that temperature for at least 30 seconds, but usually for about 5 minutes, depending in part upon the raw material and the desired cheese product.
The conventional cheese cookers used in the manufacture of process cheese products are essentially batch-type cookers in which an amount of raw material is introduced into the cooker, and heated and agitated to provide a homogenized molten mass. The molten mass is then intermittently discharged from the cooker for appropriate handling.
The design and operation of the cooker is critical in obtaining a good process cheese product. Mere heating of the cheese blend without proper agitation results in separation of the fat, thus failing to provide a satisfactory emulsion. Too much agitation during heating results in over-emulsification and a process cheese product having undesirable body characteristics. Also, maintenance of the process cheese at the cooking temperatures for an extended period of time is undesirable and burn-on of the cheese onto the sides of cookers has been a problem.
When the cheese has been cooked to the degree desired, which can be readily determined by one skilled in the art, the molten process cheese product is withdrawn from the cooker, cooled to a proper packaging or filling temperature and packaged. Packaging may take any one of a number of forms, for example, loaves or jars. Alternatively, the molten process cheese product may be formed into slices by distributing the product upon the surface of a cooled rotating chill roll in the form of a thin layer which solidifies into a sheet which is removed from the chilled surface of the roll, cut into strips and subsequently into slices followed by packaging of the sliced process cheese product.
U.S. Pat. No. 4,112,131 to Bosy, et al. is directed to a continuous in-line process for the manufacture of process cheese products. In the method, a raw material comprising a uniform blend of cheese materials is conveyed under pressure through a heating zone and a mixing zone to provide a homogeneous molten cheese mass having a moisture of from about 42% to about 75% under pressure of from about 5 psig to about 60 psig in the mixing zone at a temperature of from about 165.degree. F. to about 300.degree. F. Steam is injected directly into the cheese materials in the heating zone in a confined region and the steam is substantially condensed within the confined region, thus heating and melting the cheese materials to provide the molten cheese mass. The molten cheese mass is delivered from the heating zone to a static mixing zone where the steam is further mixed with the cheese and the temperature of the molten cheese is equilibrated. The molten cheese mass is then delivered from the mixing zone to a cooling zone having a pressure lower than the pressure of the mixing zone.
U.S. Pat. No. 2,021,899 to Schneider is directed to a prior art horizontal cooker wherein cheese particles are heated to a molten mass by a combination of steam injection and jacket heating. The horizontal cooker of the Schneider patent is provided with a screw conveyor for conveying and mixing the cheese particles as they are being heated in a batch process. At the outset of a run, the cheese particles are agitated within the cooker as steam is introduced directly into the material, while at the same time hot water is maintained in the jacket. After a molten mass is obtained, the molten mass is recirculated by means of a pump through a strainer and back into the cooking chamber. During the recirculation, the steam injection is stopped. The circulation of the molten mass is continued until the molten mass is of a thin, creamy consistency. Thereafter, the molten mass is pumped to a chamber wherein further steam is injected into the molten mass to raise the temperature to between 212.degree. F. and 245.degree. F. The material exits from the mixing chamber into a vacuum chamber where further body is developed. The vacuum is then cut and the contents of the vacuum pan are packed while hot.
U.S. Pat. No. 1,639,828 to Wheeler, et al. is directed to a prior art method of treating cheese to produce process cheese. In the process of the Wheeler, et al. patent, finely divided cheese is fed into the hopper of a cylindrical horizontal cooker. The cooker is provided with paddles to agitate the cheese particles as they progress down the length of the cylindrical horizontal cooker. Steam is introduced into the sides of the horizontal cooker by means of a heat manifold and steam jets located in the sides. As the cheese particles progress along the length of the cooker, they are heated by the steam and agitated by the paddles and a screw conveyor to provide a molten cheese mass. After cooking, the cheese mass exits from the end of the horizontal cooker into a package.
U.S. Pat. No. 1,796,445 to Doehring, et al. describes a continuous horizontal cheese cooker whereby a series of cookers are interconnected and the cheese particles are gradually heated by subjecting the cheese to different temperature conditions in the successive cookers in a continuous process.
It is evident from the extended history relating to the development of process cheese cookers that considerable research has been expended to understand the physico-chemical processes which occur during the manufacture of process cheese-type products. In order for process cheese-type products to have the correct texture, flavor and meltability attributes, they must undergo a specific temperature and shear history. A certain time at elevated temperatures is required to cook the cheese curd into a continuous phase and to eliminate uncooked cheese or "curd specks". In addition to the elevated temperature, a certain degree of agitation at specific shear values is required to facilitate the melting of the cheese and to emulsify the fat in the system.
If the shear from this agitation is excessive, the fat becomes over-emulsified causing defective texture, i.e., the process cheese-type product becomes too firm and rubbery and it will have restricted melting ability in applications such as hot sandwiches. Hence, the amount of time the process cheese-type product experiences at any given conditions of shear and temperature are critical to the finished product attributes.
Accordingly, it is a principal object of the present invention to provide a continuous method for the manufacture of process cheese-type products wherein the shear, temperature and residence time are controlled to provide a process cheese-type product having the correct texture, flavor and meltability attributes.
Another object of the present invention is to provide a method and apparatus for the continuous in-line cooking of a process cheese-type formulation, particularly by the direct introduction of steam into a flowing stream of the raw materials used to provide the process cheese-type product.
These and other objects of the invention will become apparent from the following detailed description and the attached drawings.